A conventional control unit of the same general type to which the invention relates is shown in FIG. 30. In this figure, designated at reference numeral 1 is an induction motor, at 26 a transistor inverter circuit for driving the induction motor 1 at a variable frequency, at 22 a frequency instruction generator, at 23 a function generator, at 24 a first voltage instruction generating circuit, and at 25 a PWM circuit.
The principles of frequency control in an induction motor with the control unit described above will now be discussed.
FIG. 31 is a T-type equivalent circuit for one phase of a known induction motor. In this figure, R.sub.1 indicates a primary resistance, R.sub.2 a secondary resistance, l.sub.1 a primary leak inductance, l.sub.2 a secondary leak inductance, M a primary secondary mutual inductance, .omega..sub.1 a primary frequency, .omega..sub.s a slip frequency, V.sub.1 a primary voltage, E.sub.0 a gap inductive voltage, I.sub.1 a primary current, and I.sub.2 a secondary current.
The gap magnetic flux .PHI..sub.0 is determined by the inductive voltage E.sub.0 and the primary frequency .omega..sub.1, and as time integration of the voltage indicates magnetic flux, equation (1) is established: EQU .PHI..sub.0 =E.sub.0 /.omega..sub.1 ( 1)
A current I.sub.2r which acts on this magnetic flux .PHI..sub.0 and generates a torque is an effective element of a secondary current I.sub.2, namely the same phase element of the inductive voltage E.sub.0. Accordingly, I.sub.2r is given by equation (2), as shown in FIG. 31. ##EQU1##
The torque T.sub.e generated by an induction motor is proportional to the product of the magnetic flux .PHI..sub.0 and the current I.sub.2r, so that equation (3) may be written as follows: EQU T.sub.e =K.PHI..sub.0 I.sub.2r ( 3)
Herein, K is proportional constant.
Equation (4) is obtained by substituting equations (1) and (2) into equation (3): ##EQU2##
From equation (4), when E.sub.0 /.omega..sub.1 is kept at a constant value, the generated torque T.sub.e changes according to the slip frequency .omega..sub.s. In this step, the maximum torque T.sub.max is obtained by differentiating equation (4) with respect to the slip frequency .omega..sub.s and setting the numerator at zero. Namely the maximum torque is obtained by equation (5): ##EQU3##
For this reason, the maximum torque T.sub.max has no effect on .omega..sub.1 if E.sub.0 /.omega..sub.1 is kept at a constant value.
In practice, however, as the inductive voltage E.sub.0 cannot easily be detected, generally a so-called V/F constant control system is employed in which the primary voltage V.sub.1 is made proportional to .omega..sub.1 to make the value of V.sub.1 /.omega..sub.1 constant.
In this case, in an area where the primary frequency .omega..sub.1 is low, the voltage drop due to the primary resistance R.sub.1 cannot be ignored in relation to the primary voltage V.sub.1, so that V.sub.1 is made larger beforehand by a voltage value equivalent to R.sub.1 I.sub.1 in the low frequency area.
Next, a description will be given of operations of the control unit shown in FIG. 30.
For the reasons described above, the primary frequency instruction .omega..sub.1 * outputted from the frequency instruction generator 22 is provided as an input, based on the functional relation indicated by the solid line in FIG. 32, to the function generator 23, which outputs an amplitude instruction V.sub.1 * of the primary voltage.
Then, the primary voltage instruction generating circuit 24 executes computation employing equation (6) with the amplitude instruction V.sub.1 * for the primary voltage and the primary frequency instruction .omega..sub.1 *, and outputs the primary voltage instructions V.sub.1u *, V.sub.1V *, and V.sub.1w * to the first respective coils of the induction motor 1. EQU V.sub.1u *=V.sub.1 * cos .omega..sub.1 *t EQU V.sub.1v *=V.sub.1 * cos (.omega..sub.1 *t-2.pi./3) EQU V.sub.1w *=V.sub.1 * cos (.omega..sub.1 *t+2.pi./3) (6)
Then, the PWM circuit 25 generates a base signal for controlling ON/OFF operations of a transistor (not shown) constituting a transistor inverter circuit 26 corresponding to the primary voltage instructions V.sub.1u *, V.sub.1v *, V.sub.1w *, and as a result the primary voltages V.sub.1u, V.sub.1v, V.sub.1w actually fed to the induction motor 1 are controlled so that they follow the respective instructions. For this reason, the frequency of the induction motor 1, namely, the rotational speed thereof, can be controlled according to the primary frequency instruction .omega..sub.1.
In a control unit for the conventional induction motor constructed and arranged as described above, if it is necessary to generate a large torque when running at a low speed, the primary voltage instruction V.sub.1 * must be set to a value higher enough to compensate a voltage drop, as shown in FIG. 28, to correct the voltage drop due to the primary resistance R.sub.1.
However, the primary resistance R.sub.1 changes according to a temperature, so that it is very difficult to accurately compensate the voltage drop. For this reason, when the voltage increase introduced to compensate the voltage drop as described above is smaller than the actual voltage drop, if a load torque is constantly applied to the induction motor, the torque generated when running at a low rotational speed becomes insufficient to the extent that the induction motor cannot be started. To the contrary, if the voltage increase introduced to compensate the voltage drop is too large, operation of the inverter circuit must be stopped to protect the inverter circuit from damage caused by an overcurrent due to a large primary current when running at a low rotational speed, which is very disadvantageous.
Also, even if the generated torque is constant, when the machine driven by an induction motor is changed, the total moment of inertia is different, so that the rate of change of rotational speed of the induction motor becomes different. For this reason, unless the rate of change of the primary frequency instruction .omega..sub.1 is properly adjusted, acceleration or deceleration of the induction motor cannot be performed correctly according to .omega..sub.1 *, and sometimes a large primary current may flow therein, so that operation of the inverter circuit must be stopped to protect the inverter circuit from overcurrent.
In order to solve the problems mentioned above, the present inventor proposed a control unit for an induction motor (Japanese Patent Laid-Open Publication No. 30792/1993) which does not encounter problems such as torque shortage or overcurrent, even if the value of the primary resistance R.sub.1 in the induction motor changes because of temperature, and which does not depend on the rate of change of the machine driven by the induction motor or on the primary frequency instruction .omega..sub.1 *, and which can always properly control the rotational speed of an induction motor under stable conditions.
Also, Japanese Patent Laid-Open No. 299493/1990 discloses a method for correcting a primary resistance set value. In this conventional approach, a secondary interlinkage magnetic flux is computed from a voltage and current in a motor as detected by a voltage sensor and a current sensor. The primary resistance value is corrected according to the amount of amplitude deviation between the instruction value and the computed value.
However, in this conventional approach, the voltage drop in the primary circuit is subtracted from a voltage at a terminal of the motor, and the difference is integrated with time to obtain the secondary interlinkage magnetic flux. Because of this integration with time, an accurate motor voltage is required, which means that a separate voltage sensor must be provided, which is disadvantageous. For this reason, a control method not requiring a means for computing a magnetic flux such as a secondary magnetic interlinkage magnetic flux is necessary.
FIG. 33 is a block diagram illustrating the general configuration of the above-described control unit for an induction motor (Japanese Patent Laid-Open Publication No. 30792/1993). In FIG. 33, designated at reference numeral 2 is a current detector which detects a primary current flowing in the induction motor 1, at 3 a variable frequency power exchange circuit provided in a stage before the induction motor 1, at 4 an excitation current instruction setting unit which sets up an excitation current in the induction motor 1, at 5a a no-load voltage computing circuit connected to the excitation current instruction setting unit 4 as well as to a frequency instruction generator 9 and outputting a no-load voltage instruction, at 6 an error current element computing circuit connected to the current detector 2, excitation current instruction setting unit 4 and the frequency instruction generator 9, computing primary current elements each having a phase different by 90 degrees from the other from the primary current in the induction motor 1 and the primary frequency instruction value, and furthermore computing an error current based on the excitation current instruction value and the primary current, at 7a a compensating voltage computing circuit connected to the error current element computing circuit 6, the frequency instruction generator 9, the primary resistance setting unit 10 and a primary resistance correction voltage circuit 11a and computing a correction voltage, and at 8 a primary voltage instruction computing circuit connected to the correction voltage computing circuit 7a, the no-load voltage computing circuit 5a, and the frequency instruction generator 9 and outputting a primary voltage instruction based on the no-load voltage instruction and the correction voltage.
Next a description will be given of the operations of this circuit.
A no-load voltage instruction V.sub.1q0 * is computed by the no-load voltage computing circuit 5a from L.sub.1 *, a predetermined value of the primary self-inductance previously set in a factor setting unit inside the no-load voltage computing circuit, an excitation current instruction I.sub.1d * inputted from the excitation current instruction setting unit 4, and a primary frequency instruction .omega..sub.1 * inputted from the frequency instruction generator 9 using equation (7), and is outputted to the primary voltage instruction computing circuit 8. EQU V.sub.1q0 *=L.sub.1 *.omega..sub.1 *I.sub.1d * (7)
The primary currents I.sub.1u and I.sub.1v in the induction motor 1 detected by the current detector 2 are converted on the rotating coordinate system (d-q coordinate system) rotating according to the primary frequency instruction .omega..sub.1 * and obtained as d-axis and q-axis elements I.sub.1d and I.sub.1q of the primary current above. Furthermore, the error current element I.sub.err is computed using equation (8) so that an actual value of the primary magnetic flux generated inside the induction motor 1 becomes zero when it coincides with an instruction value for the primary magnetic flux provided as a product of the excitation current instruction I.sub.1d * and the primary self-inductance L.sub.1 in the induction motor 1 from the above values I.sub.1d and I.sub.1q, a leak factor predetermined value .delta.* which is an equivalent circuit constant for the induction motor 1 previously set up in the factor setting unit inside the error current element computing circuit 6, and the excitation current instruction I.sub.1d * outputted from the excitation current instruction setting unit 4. EQU I.sub.err =I.sub.1d *-I.sub.1d +.sigma.*I.sub.1q.sup.2 /(I.sub.1d *-.sigma.*I.sub.1d) (8)
Here, .sigma.*, which is a leak factor predetermined value for the induction motor 1, is computed using equation (9) from L.sub.1 * which is a predetermined value for the primary self-inductance L.sub.1 for the induction motor 1, L.sub.2 * which is a predetermined value for the second self-inductance L.sub.2, and M* which is a predetermined value for the primary/secondary mutual inductance M, and is set up as a factor for a factor setting unit inside the error current element computing circuit 6. EQU .sigma.*=1-(M*).sup.2 /(L.sub.1 *L.sub.2 *) (9)
Then, the error current element I.sub.err outputted from the error current element computing circuit 6 above is computed by the primary resistance correcting circuit 11a using equation (10) and outputted as a correction value .DELTA.R.sub.1 for the primary resistance predetermined value R.sub.1 *. EQU .DELTA.R.sub.1 =(K.sub.RP =K.sub.RI /S)I.sub.err ( 10)
Here, K.sub.RP is a proportional gain and K.sub.RI is an integration gain.
Then correction voltage elements .DELTA.V.sub.1d, .DELTA.V.sub.1q for the d axis and q axis making the error current element I.sub.err closer to zero are outputted. Namely, the primary resistance predetermined value R.sub.1 * inputted from the primary resistance setting unit 10 is added to the correction value .DELTA.R.sub.1 for the primary resistance predetermined value R.sub.1 * inputted from the primary resistance correcting circuit 11a to make up the primary resistance estimated value R.sub.1, and the correction voltage element .DELTA.V.sub.1d for the d axis and the correction value element .DELTA.V.sub.1q for q axis are computed using equation (11) below using the d-axis element I.sub.1d of the primary current, q-axis element I.sub.1q of the primary current, error current element I.sub.err, each of which is inputted from the error current element computing circuit 6, and the primary frequency instruction .omega..sub.1 * inputted from the frequency instruction generator 9 and the correction voltage elements computed as described are outputted. EQU .DELTA.V.sub.1d =R.sub.1 I.sub.1d +K.sub.cd I.sub.err EQU .DELTA.V.sub.1q =R.sub.1 I.sub.1q +(K.sub.0 .omega..sub.1 *+K.sub.cq)I.sub.err ( 11)
Here, K.sub.cd, K.sub.cq, and K.sub.0 are proportional gains each previously set in a factor setting unit inside the correction voltage computing circuit 7a.
Then the primary voltage instructions V.sub.1u *, V.sub.1v * and V.sub.1w * are provided from the primary voltage instruction computing circuit 8. Namely the d-axis and q-axis element instructions V.sub.1d *, V.sub.1d * are computed using equation (12) below using the d-axis and q-axis correction voltage elements .DELTA.V.sub.1d, .DELTA.V.sub.1q inputted from the correction value computing circuit 7a and the no-load voltage instruction V.sub.1q0 * inputted from the no-load voltage computing circuit 5a, and the element instructions computed as described above are outputted. Furthermore the d-axis and q-axis element instructions V.sub.1d *, V.sub.1q * of the primary voltage are converted to the primary voltage instructions V.sub.1u *, V.sub.1v * and V.sub.1w * by the primary voltage instruction computing circuit 8 using the primary frequency instruction .omega..sub.1 * inputted from the frequency instruction generator 9, and are outputted. EQU V.sub.1d *=.DELTA.V.sub.1d EQU V.sub.1q *=.DELTA.V.sub.1q +V.sub.1q0 * (12)
Then, when the primary voltage instructions V.sub.1u *, V.sub.1v * and V.sub.1w * are inputted into the variable frequency power converting circuit 3, the actual value of the primary current imposed on the induction motor 1 is controlled so that the primary voltage will follow the primary voltage instruction values.
In addition to the above, reference may be made to Japanese Patent Laid-Open No. 299493/1990 disclosing an induction motor controlling method, Japanese Patent Laid-Open No. 245789/1991 disclosing an induction motor vector controlling method, Japanese Patent Laid-Open No. 261384/1992 disclosing a torque control inverter controlling method and apparatus for the purpose, the Japanese Patent Laid-Open No. 135288/1987 disclosing an induction motor magnetic vector computing device, Japanese Patent Laid-Open No. 206888/1989 disclosing an induction motor controller, Japanese Patent Laid-Open No. 21293/1984 disclosing an induction motor torque controller, and Japanese Patent Laid-Open No. 62392/1986 disclosing an induction motor vector controller.
The control unit (as disclosed in Japanese Patent Laid-Open No. 30792/1993) for an induction motor developed to overcome the problems in the conventional types of control units for induction motors is constructed as described above, so that, if R.sub.1 *, L.sub.1 *, and .sigma.* which are predetermined values for the primary resistance R.sub.1, a circuit constant for an induction motor, primary self-inductance L.sub.1, leak factor .sigma. (=1-M.sup.2 /(L.sub.1 L.sub.2)) and the like are equal to the true values, the control unit operates so that the actual value of the primary magnetic flux generated inside the induction motor coincides with the product of the excitation current instruction current and the primary self-inductance for the induction motor, and for this reason torque shortage or overcurrent does not occur, and the rotational speed of the induction motor can always be controlled under stable conditions.
In this control unit for an induction motor, rotational constants for the induction motor, namely the primary resistance R.sub.1, primary self-inductance L.sub.1, leak factor .sigma. (=1-M.sup.2 /(L.sub.1 L.sub.2)) and the like can generally be obtained from a design specification for the induction motor, or through a constant measurement test such as a resistance measurement test, a lock test, or a no load test. However, among induction motors manufactured according to the same design specification, sometimes the circuit constants may vary significantly due to variations in the manufacturing process. In such a case, or if the design specifications cannot be obtained, it is necessary to carry out a constant measurement test for each individual induction motor to be operated.
Also, sometimes an induction motor is not operated under a normal and constant excitation current instruction, but may be controlled by changing the excitation current instruction such as variable excitation, or weak excitation, but in this case in some induction motors the primary self-inductance may change significantly due to magnetic saturation. Herein, if the predetermined value for the primary self-inductance is different from the true value, a constant deviation will be generated in the error current element I.sub.err specified by equation (8). In a case where a large constant deviation of the error current element I.sub.err is generated due to an error in setting up the primary self-inductance, sometimes the output voltage may be smaller than the ideal value, which may in turn result in a torque shortage. For this reason, in this control unit for an induction motor, sometimes it is required to carry out a very complicated process such as determining the primary self-inductance change curve and storing the measured values.
Furthermore, in case of a completely closed slot-type induction motor, if the primary and secondary currents are large, sometimes magnetic saturation may occur, which may in turn cause substantial changes in the leak factor. Herein, if the predetermined value of the leak factor is largely different from the true value, especially when the load is heavy, a large constant deviation occurs the error current element I.sub.err given by equation (8). If this constant deviation of the error current element I.sub.err is largely on the negative side, sometimes the correction voltage elements .DELTA.V.sub.1d, .DELTA.V.sub.1q for the d axis and q axis given by equation (11) may become ideal values, which may in turn cause torque shortage. It is difficult to measure the change curve of the leak factor due to magnetic saturation, and with the control unit for an induction motor as proposed above, in a case where an induction motor, in which the leak factor changes significantly, is driven, the change curve for the leak factor cannot be stored, and the setting error may cause torque shortage.
It should be noted that, as far as the primary resistance is concerned, the correction can be made properly even with the control unit proposed above, and excellent control can be achieved in straight power driving, which is the most common driving state. However, no consideration has been given to the case of a regenerative load, so that, in a case where a specific load requiring a large and low-speed regenerative torque is required, for instance, in the case of driving an elevator or the like, sometimes a torque shortage still may occur.
Further, Japanese Patent Laid-Open Publication No. 299493/1990 discloses an inductive motor controlling method for correcting the primary resistance predetermined value. In this controlling method, a secondary interlinkage magnetic flux is computed from a voltage and a current in the motor detected by a voltage sensor and a current sensor, respectively, and the primary resistance value is corrected according to the amount of amplitude deviation between the instruction value and the computed value. However, in the conventional approach described above, the voltage drop in the primary circuit is subtracted from the terminal voltage of the motor, and the difference is subjected to time integration, thus the secondary interlinkage magnetic flux being computed. To carry out the integration, an accurate motor voltage value is required, and for that purpose a voltage sensor is indispensable, which is disadvantageous. For this reason, it is necessary to employ a controlling method not requiring a computing device for computing magnetic flux such as the secondary interlinkage magnetic flux.